The boy who would grow up to be America's deadliest school gunman caught the attention of Newtown High School members of staff, while teachers, counsellors and security officers helped monitor him, said Richard Novia, the director of security at Newtown School District at the time in 2007.

He wasn't feared to be a danger to others, however. Staff were instead concerned that he could be bullied or harmed by others or could harm himself.

"At that point in his life, he posed no threat to anyone else. We were worried about him being the victim or that he could hurt himself," Mr Novia told the Wall Street Journal.

Lanza, who was 20, stood out as different from an early age. He is believed by investigators to have attended Sandy Hook Elementary, the site of his massacre on Friday, before being removed and partially home-schooled by his mother.

The decision to take him out of class was one of many taken by his affluent parents as they struggled to find ways to support their troubled son.

Following their divorce, Lanza lived at his mother's colonial-style mansion, where he had two of the house's four bedrooms – one for himself and the other for the computer where he played violent video games.

Friends recalled an intensely shy but highly intelligent classmate, who wore oversized formal shirts with pens in their top pockets while others chose the youthful casual-wear typical of US schools.

"He was very thin, very remote and was one of the goths," Catherine Urso, the mother of one of Lanza's classmates, told reporters, referring to the "gothic" subculture centring on dark rock music.

Another parent recalled a description from Lanza's older brother, Ryan: "My brother has always been a nerd," he said, adding that he "still wears a pocket protector", a sheath to guard against pen leaks.

What little social interaction he had came mainly from members of the "tech club" at Newtown High School, a group that met to play computer games together.

Even among the club's members, many of whom were seeking refuge from the difficulties of adolescence, he stuck out for his awkwardness. A group photo shows him pale and ghostlike while the others smile at the camera.

Despite his interest in computers, he left almost no digital footprint and, unlike almost every other American his age, appears not to have had a Facebook account.

In the school halls he stuck out for the black briefcase he carried amid a sea of colourful rucksacks. "Adam Lanza has been a weird kid since we were five years old," Tim Dalton, a former classmate, wrote on Twitter.

Ryan Lanza is reported to have told investigators that his brother had been diagnosed with Asperger's syndrome, a disorder linked to autism whose sufferers struggle with social interaction. However medical experts have stressed it does not explain his violence.

One friend even claimed that he had a condition that meant he barely felt pain. "When he was on the baseball team, everyone had to be careful that he didn't fall because he could get hurt and not feel it," the friend told reporters.

Records show that Lanza achieved a high-school level of education but does not appear to have gone on to university or to have ever held a job.

Her son's difficulties were always at the forefront of Nancy Lanza's mind, even as she appeared to neighbours to be a bright and lively woman interested in gardening and dice games.

Marsha Lanza, her sister-in-law, said Mrs Lanza "battled with the school district" as she fought to make sure Lanza was getting the support he needed.

She taught Lanza how to shoot and would speak freely of the gun collection that he would eventually use to devastating effect.

The 52-year-old was found dead in bed five miles away from Sandy Hook school after police arrived to investigate the gunman's home. She had been shot in the head by her younger son.

Mrs Lanza described as a "survivalist" by a relative is believed to have owned at least five firearms, including the powerful rifle used by Lanza in his assault on the school and several handguns.

Friends told reporters that she was likeable but highly-strung, and had seemed anxious about coping with Lanza's developmental problems.

A slim woman with blonde hair, she was a registered Republican who enjoyed "ladies' night" at friends' homes but never hosted the evenings at her own, friends said.

She was frequently seen at My Place, a local bar, where she enjoyed drinking craft beers and discussed her love of guns.

A friend, Jim Leff, recounted on his blog that when she overheard him negotiating a loan with a friend who needed money, she promptly wrote him a cheque. He said he had thought she was "high-strung".

"I understand that it wasn't that Nancy was overwrought about the trivialities of everyday life, but that she was handling a very difficult situation with uncommon grace," he wrote.

A landscaper who worked on the garden of Mrs Lanza's sprawling home said she always emerged from a side door after he rang the front doorbell, and never let him come inside.

Her brother James Champion, a police officer in New Hampshire, said in a statement: "We reach out to the community of Newtown to express our heartfelt sorrow for the incomprehensible loss of innocence that has affected so many."

Mrs Lanza was already dead by the time American television networks began incorrectly to identify her elder son, Ryan, as the gunman responsible for the killing.

The 24-year-old was in fact 80 miles away in Manhattan, when he saw news of the killings flash across the office television. He told colleagues at Ernst & Young, the accountancy firm, that he needed to leave.

Friends, who described him as a kind and gentle young man, were already baffled. "I couldn't imagine Ryan doing such a thing," said Marsha Lanza, his aunt, who saw the early reports. "He has it together."

Making his way back to his flat in Hoboken, in neighbouring New Jersey, Ryan Lanza took to Facebook to protest his innocence. "It wasn't me I was at work it wasn't me," he wrote.

Soon 14,000 messages appeared on his page, many accusing him of the killings. Then, a tabloid confirmed police had named the wrong brother, possibly because Adam Lanza was carrying Ryan Lanza's identification.

Nonetheless, burly New Jersey police officers soon led the small, bespectacled brother away from his home in handcuffs, in front of television cameras.

He spent the next several hours talking to the FBI, reportedly describing Lanza's learning disabilities and how he had not seen his brother since 2010. Authorities described him as "extremely co-operative" and said he was not being treated as a suspect.

Ryan Lanza appeared to have had the relatively normal life of a young professional. He joined Ernst & Young after graduating from Quinnipiac University in Connecticut.

Despite not seeing his brother, he apparently remained close to his mother. His Facebook page showed he had recently been in Newtown, and he reportedly travelled to New Orleans with her for a concert.

Unlike his son Ryan Lanza, Peter Lanza apparently made no connection between the news on television and his own family and so was surprised to see a woman lingering on his front step when he arrived home from work in his Mini Cooper.

Rolling down his window, the reporter explained she was there because his son was suspected of massacring a class of children. His face twisted from "patient, to surprise, to horror," she said.

Mr Lanza, 54, has since been trying to answer the unanswerable: how his son, whom he raised in the comfort and affluence of leafy New England, could commit such a crime.

"We are in a state of disbelief and trying to find whatever answers we can," the corporate tax executive said in a statement. "We, too, are asking why." Mr and Mrs Lanza married in 1981 in their native New Hampshire, where their sons were born in 1988 and 1992. They moved to Connecticut in 1998. Mr Lanza joined General Electric and is believed to be paid about $1 million (£600,000) a year.

Though their marriage broke down in 2008, the couple were united in their determination to care for Adam Lanza, their troubled younger son.

Mr Lanza agreed to pay around $250,000 (£150,000) a year in child maintenance payments – more than his lawyers advised – in return for joint custody of Lanza and generous visiting rights.

The Republican businessman, who was described as a decent man by colleagues, moved an hour's drive away to Stamford, where he married Shelley Cudiner, a librarian at the University of Connecticut.

"Our family is grieving along with all those who have been affected by this enormous tragedy," he said. "No words can truly express how heartbroken we are."